The 2015 salary cap number isn't known yet, though it could be $141.8 million. The Packers aren't tight against it ($126 million) but they don't have a lot of room to work with. Here's the breakdown via Over The Cap.
The bottom 38 players with a cap number under $1 million count for $21,591,192. There are some very important players in this group playing on their rookie contracts (Eddie Lacy, Mike Daniels) but there are also a lot of players who are the type that come and go every season. This number will go up by a few million after the 2015 draft picks sign, but then it'll come down as players are released to start the season.
The next 7 players account for $18,643,750. These include their recent 1st round picks (Perry, Jones, Clinton-Dix) and a couple of lower priced free agents (Quarless, Starks). There shouldn't be any changes among this group, though it also includes Crosby and Masthay, and Masthay was awful last season. It depends on whether they bring in some real competition in training camp.
The next 4 players account for $18,643,750. While WR Jordy Nelson is in this group, it also includes two players who should be released (A.J. Hawk, Brad Jones) and another player who is being paid $4.25 million for no good reason (Mike Neal). Releasing those three linebackers would save them approximately $10 million, but it would also create nearly $4 million in dead money. While that's not much for some NFL teams, GM Ted Thompson never has that much dead money on his cap, so releasing them would be a change from his usual routine.
The top seven: $69,943,750. These seven players aren't going anywhere. Why even talk about releasing Peppers? He was very good last season, though he can't play every down. His release would save $7 million but create $5 million in dead money. In those terms, he's a bargain as a one-year, $7 million free agent.
If the Packers released all the linebackers that I would like them to release, then they would have about $25 million available. That sounds good, but every team is going to have salary cap room this offseason. Every free agent is going to be a bit more expensive.